Other than the Super Laser I find them to be rather fragile. Depending on your level of tech, a battle fleet handles them well. As well, when ever I discover one that someone is working on, especially Pirates, I send almost anything after it and destroy it long before it can be repaired and used. Spoiler play true, as I would rather no one have it, than my enemies having it.

Hyper deny is great for trapping legendary pirates too. Going to see about capturing this beast tonight...

Edit: lost track of the world destroyer, never to be seen again (so far). However *spoilers* the refugees from another galaxy showed up right on my doorstep and just now revealed themselves. Yay time for more conquering!